I've been teaching my five year old cousin how to play classic video games. Its been a struggle, since she can't really read yet, and most of my games are RPGs, or visual novels. She enjoys Mario, but hasn't quite learned the timing yet.
I was worried when she asked to play Sonic the Hedgehog 2, but I was actually really surprised to watch her blast through pretty much the first three levels without a game over. The game really rewards you for moving forward without hesitation, and the ring-system allows for more trial-and-error than Mario's two strike max penalty.
If she can grasp the concept of moving left instead of just right, and gets her timing down, she could possibly beat this game before too long.
3D platforming...well that's another story all together.
;;\
*Watches Mario stutter repeatedly towards a goomba inch by inch in Mario 64*